The present invention generally relates to semiconductor devices, and more particularly to a semiconductor device which has a plurality of semiconductor chips.
To satisfy demands to reduce the size of electronic equipments and the like, a semiconductor device having the a chip-on-chip structure has been proposed. The semiconductor device having the chip-on-chip structure has a plurality of semiconductor chips mounted within a single package. However, to reduce the size and weight of the semiconductor device, it is necessary to reduce the thickness of the package. When the thickness of the semiconductor device is reduced, it is necessary to carry out a resin molding without displacing the semiconductor chip mounted within the package.
FIG.1 shows an example of a conventional semiconductor device. In FIG. 1, a semiconductor device 1 has a plurality of semiconductor chips, more specifically, first and second semiconductor chips 3a and 3b. The first semiconductor chip 3a is provided at a central opening of a lead frame 2 so that a first surface 3a' having circuits formed thereon faces up. Similarly, the second semiconductor chip 3b is provided at the central opening of the lead frame 2 so that a first surface 3b' having circuits formed thereon faces up. An inner lead bonding is made so that the first semiconductor chip 3a is connected to bumps 4 at respective ends of tape leads 5a and 5b which are bent as shown. In addition, an inner lead bonding is made so that the second semiconductor chip 3b is connected to bumps 4 at respective ends of tape leads 5c and 5d.
The tape leads 5a and 5b are bent so that a second surface 3a" of the first semiconductor chip 3a confronts and is approximately parallel to the first surface 3b' of the second semiconductor chip 3b. An outer lead bonding is made so that other ends of the tape leads 5a, 5b, 5c and 5d are connected to outer leads 6 of the lead frame 2 by thermo-compression bonding. In addition, a transfer molding is carried out to make a package which is made of a mold resin 7. For example, when surface mounting the semiconductor device 1 on a circuit substrate, the outer leads 6 are bent into an L-shape or a J-shape.
A semiconductor device exists which has the first semiconductor chip 3a arranged so that the second surface 3a" thereof faces up, and the surfaces 3a' of the first semiconductor chip 3a and the surface 3b' of the second semiconductor chip 3b confront each other.
Next, a description will be given of essential steps of a method of producing the semiconductor device 1 shown in FIG. 1, by referring to FIGS. 2A and 2B.
As shown in FIG. 2A, the lead frame 2 and the semiconductor chips 3a and 3b of the semiconductor device 1 are connected to the tape leads 5a, 5b, 5c and 5d, and the semiconductor chips 3a and 3b and their peripheral parts are set within a cavity which is formed by upper and lower dies 8a and 8b. Then, the mold resin 7 is injected via a gate 10, so as to carry out a transfer molding. As shown in FIG. 2A, the semiconductor chips 3a and 3b will not be displaced by the injection pressure of the mold resin 7 at the initial stage of the injection process.
But the semiconductor chip 3a is only supported on the outer leads 6 by the tape leads 5a an 5b. In addition, the tape leads 5a and 5b have a relatively poor rigidity because these tape leads 5a and 5b are made of a polyimide tape which is approximately 125 .mu.m thick and has a copper film pattern having a thickness of approximately 35 .mu.m adhered thereon, for example. Since the tape leads 5a and 5b having this relatively poor rigidity are bent in the shape shown in FIG. 2A, the tape leads 5a and 5b may be deformed by the injection pressure of the mold resin 7 as the injection process progresses, thereby displacing the semiconductor chip 3a.
Similarly, the rigidity of the tape leads 5c and 5d is also relatively poor, and the semiconductor chip 3b may be displaced slightly as the injection process progresses.
Accordingly, the positions of the semiconductor chips 3a and 3b, the viscosity of the mold resin, the injection pressure of the mold resin 7 and the like are controlled so as to prevent the semiconductor chips 3a and 3b from being displaced during the injection process.
However, according to the conventional semiconductor device 1, the tape leads 5a through 5d are deformed as shown in FIG. 2B when the mold resin 7 is injected to a certain extent if the production precision of the tape leads 5a through 5d includes variance, even if the variance in the injection pressure of the mold resin 7 is sufficiently controlled, thereby causing the semiconductor chips 3a and 3b to be displaced. As a result, there are problems in that the second surface 3a" of the semiconductor chip 3a may make contact with the tape lead 5d, and the tape lead 5a may make contact with a top edge portion of the semiconductor chip 3a to cause a short-circuit, as shown in FIG. 2B.